In most of the cultivars growing in California, harvest date is determined by skin color changes that are described for each cultivar. A color chip guide has been designed to determine maturity for each cultivar.

A three tier maturity system is used in California
1) US-Mature (Minimum Maturity)
2) Well-Mature
3) Tree Ripe

Measurement of fruit firmness is recommended for cultivars where skin ground color is masked by full red or dark color development before maturation.

Maximum maturity: Flesh firmness, measured with a penetrometer with an 8 mm-tip, can be used to determine a maximum maturity index, which is the stage at which fruit can be harvested without suffering bruising damage during postharvest handling.

• Plums are less susceptible to bruising than most peach and nectarine cultivars at comparable firmness.